I looked up both names: Both are gaining popularity, but Jocelyn is much more in use right now.

Jocelyn has French and German origins, and can mean "playful" or "light-hearted". However, it can be pronounced either Joss-lyn or Joss-uh-lyn, so that might be a factor if you dislike the name being mispronounced. A similar option that is also pretty is Jocelyne "Joss-uh-leen"

Juliet has English, French and Italian roots, and can mean "woman with light hair" or youthful. It isn't likely that Juliet is going to mispronounced though!